It’s the PETG. Specifically, it is the softening on the following parts:
I would recommend the following Y-axis mod for the MK4. I am still running this even on the MK4S, even though the Y-axis was modified on the MK4S.
Edit: Print those in ASA and replace them, tighten all the belts as usual, and I think your printer will be fixed.
My money is on the PETG starting to loosen and you losing tension on the belts.
You have the MK4 in an enclosure–I did too. At some point I stopped being able to print flexibles, but ASA/PCCF was still fine. Prusa support told me that the idler was getting loose and my hotend was losing grip on the fillament.
I had another print where there was a nearly 2-3cm shift in the layers between layer 5 and layer 80. Some of my belts had absolutely lost the plot. I reprinted everything in ASA or PCCF, and while upgrading to the MK4S rebuilt the printer with all ASA/PCCF parts. No problems since.
Interesting that the extra 10° makes such a difference for ASA and ABS.
I recently started printing with ASA in my enclosed MK4. I might have to try this.
Not sure what you need to get approved. Buying a roll of ASA? It’s part of printer maintenance, and making it more reliable in the long run.
And if you don’t have ABS/ASA on hand… why is the printer in an enclosure?